Transmitting and amplifying system



May 9, 1939. L. A. MOLIN El AL v TRANSMITTING AND AMPLIFYING SYSTEM 3 Sheets-Sheet 1 Filed Sept. 26, 1936 [0m (ZMoZuz. T/ZOdOiQ lefiarorz, 5

y 9, 1939- A. MOLIN ET AL 2,157,140

TRANSMITTING AND AMPLIFYING' SYSTEM Filed Sept. 26, 1936 5 Sheets-Sheet 2 04143 (ZMOZUZ, Theodore LeBarOTZ-X ?Z m X W y 9, L. A. MOLIN ET AL 2,157,140

TRANSMITTING AND AMPLIFYING SYSTEM Filed Sept. 26, 1936 3 Sheets-Sheet 3 UUU 04415 (Z. M01672, Theodore LeBaram Patented May 9, 1939 UNITED STATES PATENT OFFICE TRANSMITTING AND AMPLIFYING SYSTEM Louis A. Molin and,Theodore Le Baron, Chicago, Ill.

Application September 26, 1936, Serial No. 102,821

14 Claims.

5 In its illustrated form the entire circuit may operate from an ordinary direct or alternating current supply with a single multipurpose tube, one portion of which serves as a rectifier and the other as an amplifier; two filters being provided, one for smoothing out the rectified current and the other between the microphone and the amplifying section 'of the tube for permitting inde-' pendent modulations of the direct current which passes in series through the microphone,a pri- 15 mary transformer coil the amplifying tube and the loud speaker.

An important object of the invention is to provide a simplified circuit for amplifying sound in 2Q operated by having the same current flow through both, although the modulations of this current are greatly amplified between the microphone and the speaker.

Another object of the invention is to provide a simplified interoffice communication system and one which will beat least as satisfactory asthe more complicated and expensivesystems heretofore known, and one which will do away with the batteries required by such'systems heretofore. 1m Another object of the invention is to provide a simplified call system or public address system.

Still another object of the invention is to provide improved features of interconnecting oflice communicating or paging units.

Additional objects will be apparent from the following description and from the drawings, in which:

Fig. 1 is a perspective view of a unitembodying one form of this invention.

Fig. 2 is a rear view of the structure shown in Fig. 1.

Fig. 3 is a circuit diagram of the structure shown in Fig. 1, together with an extension station connected therewith, and indicating the connections to a distant unit.

Fig. 4 is a fragmentary circuit diagram of a filter unit which may be replaced for one shown in Fig. 3. 3 5 'Fig. 5 is a fragmentary diagrammatic illustratlon showing a modification in which an earphone may be connected in place of the loud speaker.

Fig. 6 is a diagrammatic illustration of a switch- 5 board for connecting a unit such as that shown in Fig. 1 with anyone of a plurality or telephone sets; and I z I Fig. 7 is a diagrammatic illustration of a circuit similar to that shown in Fig. 3, but using two tubes and connected as for a call system or a public address system.

Although this invention may take numerous forms, only a few have been chosen for illustration. The formillustratedparticularly in Figs. 1 to 3 is shown as used in an intercommunicating of similar units system especially suitable, \for example, for use in oilices in which one unit is positioned on the executives desk and another positioned on the desk of some other person in-the organization.

The unit includes a case H carrying a microphone l2 for picking up and transmitting the sound, waves of the person talking near this unit, and a loud speaker l3 for reproducing the voice of the speaker at the distant unit. The case is, of course, provided with a suitable opening or openings l4 for passing sound to the microphone and a suitable opening [6 for emitting the sound producedby the speaker. The unit is preferably also provided with a plug-in power cable I! for plugging into a convenience outlet which may be the customary 110vo1t circuit, either A. C. or D. C. A .switch I8 is provided for turning the power on and oh, and a pilot light l9 visible through a lens 2| in the casing is provided for indicating when the power is turned on. Another switch 22 is provided for connecting the unit for talking or listening alternatively.

The arrangement of the various parts of the circuit can be seen in Fig. 2, but the circuit as a whole will be made more clear by reference to Fig. 3, in which the unit of Figs. 1 and 2 is'enclosed within the dotted line i l (representing case II); An identical distant unit is represented by H" and a simple local extension unit by 91. From the cable l1 one wire 26 leads to a resistance unit which may comprise parallel resistance elements 21. A wire 28 connects-these resistance elements 21 with the heaters 29 of the multipurpose tube 30, which is known on the market as type 12-.-A'7. If desired, the wire 28 maybe connected to the resistance elements 2! through ad justable taps 32. It should also be noted that the pilot light l9 may be connected across a portion of the resistance by the adjustable tap 33. The other side of the heaters 29 is connected to the supply cable I! through a wire 34 and the switch l8. I8 is closed, the heaters 29'are heated to heat the cathode 36- to maintain theset in condition for operation.

It is thus seen that whenever the switch one-half of the cycle if connected to an alternating current line. Wire 34 is connected by wire 38 with plate 48 so that when wire 34 is positive an electronic current can be drawn from cathode 36 to plate'40, the cathode 36 being supplied with current through the wire 4|. Since current can not fiow in the other direction from plate 40 to cathode 36, the current in wire 4| is rectified in accordance with well known principles.

Also in accordance with well known principles, a filter 42 is provided for smoothing out the flow of current which in wire 4| is pulsating if the unit is connected to A. C. current, so that in wire 43 the flow of current may be smooth. Wire 43 is connected to the coil 44 of speaker I3 which is a magnetic type of speaker. This coil is connected by wire 46 to plate 4'I'of the pentode section of the tube 38. Plate 4! draws current from cathode 48 which is connected to ground in a manner to be described so that the plate circuit is completed through ground to wire 26.

The screen grid of the tube 30 is connected by wire 52 to wire 43. A portion of the total cathode current of the pentode section passes to the screen grid and thence to wire 43, maintaining the screen grid at positive potential with relation to the other elements of the tube. This screen grid current flows through the microphone circuit but not through the speaker, hence providing a desirable surplus for the microphone cir-' cuit.

coil .56 of transformer 51, one side of which may of the transformer 51 which in turn is controlledby the microphone I2 of the remote unit.

The microphones I2 and I2 are preferably a carbon resistance type of microphone and therefore a normally constant .current should be provided to be passed through the microphone I2 and the primary coil 6I in series 'to be modulated by the microphone I2 in'order to modulate the voltage on control grid 53 which in turn causes a reproduction of the sound by the speaker I3. It may be mentioned at this point that each of the units II and II' is provided with four terminals A, B, C, and D. The terminals A and B of one unit are connected to the terminals C andDiof the other unit, and the terminals A and B' of the second unit are connected to terminals C and D of the first unit.

The current for the microphone and primary coil 6| may be called the microphone currentand is provided by virtue of the fact that the microphone and coil 6| are connected in series between the cathode 48 and ground. This circuit may be traced from cathode 48 through wire 63, resistance II of filter 64, wire 66, coil 6|, wire 61, terminal C, wire 68 extending between units, through local circuits adjacent microphone I2 to be described,

through the distant microphone I2, back through line wire 69, terminal D and wire III to ground.

It will be observed that when no sound is being picked up by the microphone I2, the same current passes through the microphone I2, through the primary coil 6|, through the pentode section of the tube to plate 48 and throughthe coil 44 of the speaker. However, when soundis picked up by microphone I2, this'modulates the microphone current which, as stated in the previous sentence, is the same current that passes through the speaker circuit. The current passing through the speaker circuit, however, has greatly amplified modulations as a result of the action of the control grid 53. To prevent conflict between the amplified modula'tions in the speaker circuit and the unamplified modulations in the microphone circuit, the filter 64 is provided. As

shown in Fig. 3, this filter comprises a resistance 12, through which the microphone current flows to the cathode 48, and the condensers I3 and I4 which smooth out the pulsations on the opposite sides of the filter to prevent conflict between them. a The condenser .14 is a by-pass for the audio-frequencies handledby the tube 38. Talking into ,themicrophone I2 causes current variations on the microphone side of filter 64. These variations are induced into the secondary 56 and fed to the grid 53 to be amplified and reproduced in the speaker. The condenser I3 acts to smooth out these pulsations or variations and prevents their interference with the normal action of the tube.

In place of the resistance 'I2-there may be provided an inductance-I6, as seen in Fig. 4. The inductance I6- may be the primary coil 6| of the transformer, if desired, in which case a single condenser on the cathode side of the transformer servesthe dual purpose of-by-passing audiofrequencies of the cathode plate to groundand of furnishing the additional current necessary for operation of the microphone. Such anarrangement is not preferred because of the desirability of using the resistance I2 to balance the various line loop resistances, and it would be undesirable to wind different sizes of transformersor chokes for this purpose. This completes the fundamental circuit description, but there are various additional features used in conjunction with this fundamental circuit -which should,also be describedl The most universally used of these additional features is probably the listening and talking switch although its form and operation vary in different systems.

In the form shown in Fig.3, the listening and talking switch 22 has two contact arms 8| and 83. In most units the contactarm 8| will be in a position to render the microphone ineffective, it being shown normally open at the contact 82 for breaking the circuit to the microphone I2. The purpose of this is to prevent howling. If the microphone I2 and speaker I3 are both functioning at both ends of the line, howling may result in that sound producedv by the speaker I3 will be picked up by the adjacent microphone and reproduced and again picked up at the remote unit. Obviously the contact arm 8| could close a shunt across the microphone instead of opening the circuit.

'. As shown in Fig. 3, the contact arm 83 of switch 22 is normally open and ineffective. This leaves the speaker I3 in condition to be actuated by the microphone at the remote point. When it is desired to talk over the microphone H, the key 22 is operated to close the contact 8|, thus placing the microphone I2 in the circuit of terminals A and B which, being connected to terminals C and D of the remote unit, connect the microphone I2 to operate the speaker at the remote unit just as the microphone I2 operates the speaker I3 in the manner previously described. At the same time that contact 8| closes: contact 83 closes, thereby closing a shunt through wires 86 across the coil 44 of the speaker I3 to prevent howling in case both the microphone and the speaker at the other unit are operative. Of course, at either one of the units the contact arm 83 or its adjacent contact 84 may be omitted so that the speaker at that end will never be disconnected.

In Fig. 5 a modification is illustrated in which an auxiliary earphone is provided, together with a switching arrangement which will permit disconnecting the speaker so that the person having the earphone can listen with privacy. It will be understood that this is a fragmentary'showing in which the remainder of the unit would be the same as the main unit shown within the dotted line II in Fig. 3. The lead 46' instead of extending from the plate 41 'of tube 30 would extend to contact arm 83' of an earphone switch 94. Contact 83 is normally in contact with 84' to connect the speaker in the circuit as illustrated. When-the earphone switch 94 is operated, contact arm 83' breaks with contact arm 84' to disconnect the speaker and connects with contact arm 95 to connect the earphone I00 through wires 96 and resistance 91, which is of proper value for limiting the current in the earphone to a satisfactory value, the earphone being perhaps of a thousand ohms resistance. At the same time that the earphone switch 94 is operated to connectthe earphone in place of the speaker, it operates the contact arm 96 to break the contact I8 in order to break the shunt for the microphone so that the microphone will be con- 'nected. There is no danger of howling while a person is holding the earphone to his ears and so both the microphone and earphone may be connected at the same time as in the ordinary telephone.

When it is not desired to use the earphone, the

talking switch 22 may be operated in the usual I manner having substantially the same efiect as in Fig. 3, the contact 83 closing when the switch operates for talking to shunt across the speaker by closing against the contact 84, thus rendering the speaker inoperative to prevent howling. The

contact arm 82' has been shown normally closed against a contact 8I' for closing a shunt across the microphone when the switch is not operated for talking. Shunting across the microphone, of

1 course, has the same ultimate efiect, as disconnecting the microphone as shown in Fig. 3.. In connection with switches 22 and 95 it should be understood that these switches would usually be operated optionally by a single key.

Another additional feature which may be used "with'the unit of Figs. 1 and 2 is an extension unit shown diagrammatically at 9I in Fig. 3. In

this extension unit there is no source of power I provided, the source of power in the main unit being utilized. A speaker I3X, a microphone Thus, it is apparent that when a person at a.

distant station talks into the microphone I2,

after pressing his talking key, his voice will be reproduced both by speakers I3 and I3X. When an operator at the extension station 9| desires .to reply, he merely shifts the key 22X which closes contact arm 8IX against the contact 82X to connect the 'microphone I2X in the circuit of the distant unit. At'the same time contact arm 83X will.c1ose against contact 84 to shunt the coil 44X so as to prevent howling.

It should be understood that in most installations, the extension unit 9| will not be desired, but Fig. 3 illustrates how simple it may be and how easily it may be attached. Where it is expected that extension units may be desired, terminals E, F, G, and H may be provided on the main unit for this purpose and these terminals as well as the terminals A, B, C, and D may be of the plug-in type, if desired.

In Fig. 6 is illustrated a'switchboard IOI for connecting a unit II to any one vof a plurality of difierent units, consisting of either units II similar to the unit II or telephone instruments I02. In the case of telephone instruments special signalling means may be provided, if desired, and any arrangement may be made for providing power for talking to the user of the telephone instrument, since such instruments do not normally include a source of power. The switch board IOI may be provided with suitable terminals I03 for connection with the terminals A and B of the unit I I. To each of the terminals I03 a connection may be made to one of a bank of switches I04 which operate in unison to connect the four terminals I03 to four contacts,

, such as the contacts I00 which are all connected to one of the units II' with the'result that the units II and II are connected as shown in Fig. 3.' i t In Fig. 7 there is shown an amplifying circuit in which two tubes 30 and I30 are used in a generally parallel arrangement for increasingthe output available for the speakers. This feature of parallel tubes may be used in the units shown in the preceding figures, if desired, although-the two tubes will not ordinarily be necessary under the circumstances there illustrated. the two tube circuit has been illustratedas used in a call system suitable, for example, for paging a salesman in a store or a foreman in a plant.

For this purpose there may be provided a plurality of microphones I2" located at difierent points of convenience and each having associated therewith a normally open switch III so that by closing the switch at any microphone that microphone may be connected into the circuit. These In Fig. 7

microphones may be connected acrosswires IIZ and I I4 which may be plugged in at the terminals C and D. There may likewisebe a number of loud speakers I3 and H3 connected in parallel across Wires H6 and III. The speaker I3 may be built into the unit and the other speakers may be connected to an extension line which may be plugged in at the terminals G and H. Each of the loud speakers may be providedwith a transformer II8 which may have a ratio of 1 to 1,

its 'sole purpose being to keep the speaker windings 44 out of the plate circuit if this be desired. The fundamentals of this circuit are the same as in Fig. 3 even though two tubes 30 and I30 are provided. This may best be made clear by tracing the rectified portion of the circuit start- 7 H2, through whichever microphone I2 has its switch closed, back to the unit through wire III, through primary coil I 6| of transformer I51, through resistance I12 of the special filter I64, through the cathode 48, thence by electronic discharge to plate 41 and wire II6, through the loud speakers (or the primary coils of the transformers II8 when such transformers are provided) back through wire In, through the filter 42 to wire M and to the rectifying portion of tube I30. This rectified circuit is obviously substantially the same as that shown in Fig. 3, the same current passing through the microphones and the loud speaker circuit, interference between these two portions being prevented by filter, I64.

There will normally be several loud speakers in parallel and it is for this reason and also to provide more volume in the loud speakers that the two tubes and I30 are provided. The pentode sections of these tubes are connected in parallel. Thus, the secondary I56 of transformer I5! is connected to both control grid 53 of tube ally in use.

30 and control grid I53 of tube I30. Resistances I55 and I58 correspond to the resistances 55 and 58 of Fig. 1. Likewise, screen grids 5| and I5I are both connected to wire I. thermore, the electronic flow through the pentode sections of the two tubes is in parallel for cathode I48 is connected by wire I50 with cathode 48, and plate I" is connected by wire I60 with plate 41.

The rectifier sections of the two tubes 30 and I30 are connected in an arrangement which doubles voltage when connected to an A. C. source of power, and since this arrangement is conventional it need not be described in detail.

The circuit of Fig. 7 with its two tube.rectification' and with its two pentode sections in parallel provides ample power for operating several loud speakers, and the circuit is therefore adequate either for a call system or for a public address system. Both the circuit in Fig. '7 and the circuit of Fig. 3 combine the characteristic of extreme simplicity and low cost with high dependability and quality. This is largely accomplished by virtue of utilizing the same current in the microphone circuit and the loud speaker circuit, interference between the two portions of this combined series circuit being prevented by filter I64.

An important feature of both of the circuits of Figs.'3 and '7 is that the switch for themicrophone is in the plate circuit ofthe tube and keeps this circuit open except when the system is actu- This increases the life of the tubes amazingly, it being estimated that they will last perhaps 300 times as dong as they would otherwise. The system may be turned on, with the tubes heated, for eight hours or more aday, but the electronic circuit can fiow'from the-cathode only during the time that the microphone circult is closed for talking.

Although the various resistances, inductances, condensers, and the like may havea wide range of values and flexibility in this respect is one important feature of the invention, the following suitable values'may be mentioned as one example of values which will be satisfactory.

In filter unit 42 the inductance maybe a 500- ohm, 15-henry inductance. The condensers may be B-microfarad, GOO-volt condensersf although in the'Fig. 3 form -volt condensers will be sufllcient,and in view of the half wave character Fur of the rectification it is better that the righthand condenser be at least 16-mic'rofarad.

The resistance of the speaker-coil or of the primary coil of the output transformer, if one is provided, together with-the resistance of the remainder of the speaker circuit should be such that the plate current will not exceed 15-mills for the amplifier section of each tube. It follows that if several speakers are connected in-parallel the individual resistances should be higher than when only one speaker is connected. Furthermore, in Fig. '7 where two tubes are used, the total plate current may be 30-mills.

In the filter unit 64 the resistance I2 may be of 600 to 1000 or more ohms depending on the microphone circuit, and the relation of the resistance 12 to the current flowing through it should be such as to provide a proper grid bias. The condensers may be 50-volt condensers having a capacity of from 3 to 25 microfarads, the electro:

lytic type being preferred. The resistance of the microphone and the microphone leads including the wires between stations should be such that the external loop resistance does not exceed 1000- ohms. The-vo1ume control resistance 55 may sistance 58 may be anything up to about 1500- -ohms. It may also be noted that the resistances I2 and 58 may be of extremely small current carrying capacity, a one-watt resistance being ample for resistance I2 and a e-watt being ample for resistance 58.- The power resistances 27, on the other hand, should together be capable of carrying 5Il-watts continuously and should have a resistance of '750-ohms, although if a fixed resistance is used it probably will not need to be so high.

In Fig. 7 condensers I11 and I19, which are in the nature of voltage doubling condensers, should be 20-microfarad, 200-volt electrolytic condensers.

The disclosures of this application are illustrative and the invention is not to be limited by them. In fact, if modifications or improvements are not at once obvious, they may be devised in the course of time to make additional use of the broad ideas taught and'covered by this application. The claims are intended to point out novel features and not to limit the invention except as ,may be required by prior art. For example, it

uum tube including cathode, plate, and control grid, a loudspeaker having its operating co'il connected to said plate, a microphone and a primary coil of a transformer connected in series to said cathode, said control grid being connectedto the secondary or said transformer, and filter means between said cathode and said microphone and primary coil for preventing confiict between portions of the circuit on opposite sides of said filter means.

2. A single tube sound, amplifying system comprising a tube having a rectifier section and anamplifying section; a series circuit including said rectifier section, a smoothing filter, a speaker operating coil, the amplifying section, a.- conflict preventing filter, a primary coil of a transformer, and a microphone, means for connecting said circuit to a source of alternating current, and a circuit in parallel therewith for heating the cath- Odes of said tube, said amplifying section including a control grid and a secondary coil for said transformer connected to said control grid.

3. An intercommunicating system including a plurality of vunits, each unit including a microphone adapted for connection to one other unit, and a loud speaker adapted for connection to the other unit and at least one of said units including an amplifying circuit including means for connecting the microphone of one unit and the loud speaker of the other unit in series through an amplifying tube, and filter means for preventing conflict between the microphone portion of the circuit and the loud speaker portion of the circuit.

4. A sound amplifying system including a vacuum tube including cathode, plate, and control grid, a loud speaker having its operating coil connected to said plate, a microphone and a pri-- mary coil of a transformer connected in series to said cathode, said control grid being connected to the secondary of said transformer, filter means between said cathode and said microphone and primary coil for preventing conflict between portions of the circuit on opposite sides of said filter means, a normally open switch in series with said microphone whereby the life of the tube is extended by maintaining the cathode circuit open except when someone is talking, and means for heating said cathode even when said switch is open.

5. A sound reproducing-system including an amplifying tube having a cathode, a cathode heater, a control grid .and a plate therein, a reproducing circuit connected to said plate, a circuit including a resistance type modulator connected to said cathode, a source of direct current for causing a current to flow throughsaid modulator to said cathode and through said reproducing circuit in series, means for impressing the modulations of said modulator on said control grid, a normally open switch in series with said modulator for disconnecting said cathode from said source of direct current except when" said system is in use while permitting said cathode to remain heated ready for use whereby the life of the tube is greatly extended.

6. A- translating and amplifying circuit including a device for translating sound impulses into electrical modulations and of a nature requiring the'passage of current therethrough, a transformer'including a primary winding connected to said translating device and a secondary winding, an electronic tube including cathode, control grid and plate; said cathode being connected in series with said primary winding and said grid being connected to said secondary winding, and means for heating said cathode and applying a voltage to said plate and said translating device for causing a current to pass through said translating device, said primary winding, and said cathode to said plate.

7. A translating and amplifying circuit including a devicefor translating sound impulses into electrical modulations and of a nature requiring the passage of current therethrough, a transformer including a primary winding connected to said translating device and a secondary wind ing, an electronic tube including cathode, control grid and plate; said cathode being connected to said primary winding and said ,grid being connected to said secondary winding, means for heating said cathode and applying a voltage to said plate and said translating device for causing a current to pass through said translating device,

said primary winding, and said cathode to said plate, anda filter interposed between said primary winding and said cathode for decreasing the modulations impressed on the current by the translating device.

8. A sound amplifying system including a vacuum tube including cathode, plate, and control grid, a loud speaker having its operating coil connected to said plate, and a microphone and a primary coil of a transformer connected in series to said cathode, said control grid being connected to the secondary of said transformer, whereby the same electrons pass through the microphone and theprimary coil to, the cathode,

are discharged from the cathode to the plate and pass through the operating coil of the speaker to enable the microphone and, the plate circuit of the tube to be jointly operated from a single source of high Voltage direct current.

9. A translating and amplifying circuit including a device for translating impulses into electrical modulations and of a nature requiring the passage of current therethrough, a transformer including a-primary winding connected to said translating device and a secondary winding, an electronic tube including cathode, control grid and plate; .said cathode being connected in series with said primary winding and said grid being connected to said secondary winding, and means for heating said cathode and applying a voltage to said plate and said translating device for causinga current to pass through said translating device, said primary winding, and said cathode to said plate.

10. A translating and amplifying system including -an initial translating device adapted to modulate a current, a second translating device, a coil for controlling said second translating device responsive to current modulations, an amplifying tube including a plate and a cathode connected in a. plate circuit including in series, said coil, the initial translating device and a source of current, and control means acted upon by the plate circuit in response to the initial translating device for impressing a charge on the grid to'control the flow of electrons in the plate circuit through the tube and said coil.

11. A translating and amplifying system including an initial translating device adapted to modulate a current, a second translating device, a coil for controlling said second translating device responsive to, current modulations, an amplifying tube including a plate and a cathode connected in a plate circuit including in series, said coil, the initial translating device and a source of current, "control means acted upon by the plate circuit in response to the initial translating device for impressing a charge on the grid to control the flow of electrons in the'plate circuit through the tube and said coil, and filter means interposed in the plate circuit between said last named means, and said initial transplifying tube including a plate and a cathode connected in a plate circuit including in'series, said coil, the initial translating device and a source of current, control means acted upon by the plate circuit in response to the initial translati'ng device for impressing a charge on the grid to control the flow of electrons in the plate circuit through the tube and said coil, and filter means interposed in the plate circuit between said last named means, and said initial translating device on one side and the cathode and said coil on the other side for permitting fluctuations in the flow of current through the tube and coil substantially independently of fluctuations of current through the initial translating device, said filter means including a. condenser connected ac'ross the initial translating device and the point of actuation of the control means by the circuit.

13. A translating and amplifying system including an initial translating device, a second translating device, a coil for controlling said second translating device, an amplifying tube including a plate connected to the coil, a cathode within the tube, a second coil connected to the cathode, means for causing current to flow 'nature requiring the passage of current therethrough, a transformer including a primary winding and a secondary winding, an electronic tube including cathode, control grid and plate, and a second translating device for producing sound waves and controlled by a coil, said first named translating device, said primary winding, the cathode and plate of the tube, and the coil "all being connected in a series circuit with means for applying a voltage thereto for causing a current to flow through said first named translating device, said primary winding, from said cathode to said plate, and through said coil in series, and said control grid being connected to said sec! ondary winding.

LOUIS A. MOLIN.

THEODORE LEBARON. 

